Rock bolting is an important means for the stability control of geotechnical projects such as mines, tunnels and side slopes. In the research and application of rock bolting, scientists and technicians have developed numerous types of rock bolts and successfully solved a lot of problems in support engineering. However, most of existing rock bolts can hardly adapt to rock mass engineering which features high stress, large deformation and impact proneness. According to the characteristics of stress and deformation of such rock mass engineering, an effective form of rock bolt should have high strength and reasonable yielding property. The basic requirements on such kind of rock bolt are to provide adequate deformation capacity to adapt to large deformation of surrounding rock and meanwhile to provide a relatively high anchoring force continuously in the process of deformation. Currently, main rock bolts of such kind commonly in use include the extensible constant-resistance rock bolt and the rock bolt with yieldable tube at its outer anchor end.
The extensible constant-resistance rock bolt consists of the constant-resistance device, rod body, anchor plate and nut. Among the components, the constant-resistance device comprises the constant-resistance casing tube and constant-resistance body. The inner surface of the constant-resistance casing tube and the outer surface of the rod body adopt the threaded structure. The constant-resistance device is set on the tail part of the rod body, with the anchor plate and nut installed orderly on the tail part of the constant-resistance device. When an axial force applied on the rock bolt reaches its designed constant resistance, the constant-resistance body will slide with friction along the inner wall of the casing tube, and the constant-resistance property will remain unchanged during such slide. Such rock bolt can provide relatively good yielding and anti-impact effect under certain conditions, but it requires bonding at the anchor rod head and the constant-resistance casing tube separately, thus bringing about much difficulty to rock bolt installation; and once the bonding section breaks or loses, the entire rock bolt will fail. Therefore, such rock bolt is defective with difficult installation and low reliability.
The rock bolt with yieldable tube comprises the high-strength rod body, yieldable tube, anchor plate and nut. Compared with ordinary rock bolts, this rock bolt has an additional yieldable tube at its outer anchor end to realize the function of yielding under certain pressure. Depending on different conditions of surrounding rock, the yieldable tube can be designed and manufactured in different sizes. This kind of rock bolts is good in impact resistance. However, on one hand, its yieldable tube is located at the outer anchor end so the length of the yieldable tube that determines the maximum yielding deformation is greatly restricted; on the other hand, it cannot adopt the mode of full-length anchorage in order to give play to the yielding effect, so the entire rock bolt will fail once the bond anchorage section breaks or loses. Therefore, such rock bolt is problematic in its restricted yielding deformation and low reliability.
In summary, all existing types of rock bolts will encounter problems when applied in large-deformation and/or impact-prone rock mass engineering under high stresses, so it is necessary to research and develop a kind of rock bolt which is better in adaptability and reliability.